Where are you trying to add it from?

I noticed a wide range of choices on the Mycroft site: http://mycroftproject.com/youtube-search-plugins.html

I see, the one offered by Youtube can't be added from the menu item. Hmmm, why is that?

Seems to be working when copied to the searchplugins folder.

• https://developer.mozilla.org/en/docs/Creating_OpenSearch_plugins_for_Firefox

<?xml version="1.0" encoding="UTF-8"?>
<OpenSearchDescription xmlns="http://a9.com/-/spec/opensearch/1.1/">
  <ShortName>YouTube Video Search</ShortName>
  <Description>Search for videos on YouTube</Description>
  <Tags>youtube video</Tags>
  <Image height="16" width="16" type="image/vnd.microsoft.icon">http://www.youtube.com/favicon.ico</Image>
  <Url type="text/html" template="http://www.youtube.com/results?search_query={searchTerms}&amp;page={startPage?}&amp;utm_source=opensearch" />
  <Query role="example" searchTerms="cat" />
</OpenSearchDescription>

Yes, is it a youtube problem?

@cor-el I'm not sure what to do with that, should I save that code into a XML file and place it inside the searchplugins folder?

Copying this text to a youtube.xml file and copying this file to the searchplugins folder worked for me to see the search engine in the search engine drop-down list.
You need to create this searchplugins folder.

You can use this button to go to the Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)

Thank you so much cor-el! It's working perfectly